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updated 9/11/2011 4:41:09 PM ET 2011-09-11T20:41:09

Threatening messages have been posted on the White House Facebook page amid heightened alerts surrounding a possible 9/11 terror threat.

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"We'll come back U.S.A. One day only 11/9/2011," says one message, featuring a photo of Osama bin Laden, using the date/month formula to reference Sept. 11.

"We'll come to u white house sooooooooooon," says another.

"We'll come back 11/9/2011 to kill u all," a third posting reads. The posts appeared briefly and were taken down.

Secret Service spokesman Ed Donovan said the agency has referred the messages to its internet threat desk.

"They process it, make an assessment and determine what the next step is," Donovan said.

New York City and Washington, D.C., have tightened security after intelligence collected from overseas indicated a possible threat involving car bombs, as well threats to bridges and tunnels. The information indicated that three men would travel from Pakistan to the U.S. to carry out an attack.

The threat has been described as "specific" and "credible" but not confirmed.

FBI Joint Terrorism Task Force officials across the country have been conducting interviews to eliminate people who could match travel patterns of terrorists.

A government official stresses security officials do not know if any terror suspect entered the United States at all, and the questioning of travelers across the country is precautionary and is meant to rule people out.

Law enforcement and intelligence sources said Sunday that the scrutiny of airline passenger records has turned up well over 100 names of interest, and those people have each been interviewed. None was thought to be part of any terror plot, though officials decided to add some of the names to terror databases, simply because of their travel patterns, for the sake of future scrutiny.

And around New York and Washington, officials say the FBI has found no sign of unusual purchases of chemicals needed to make car bombs.

A joint FBI-Department of Homeland Security bulletin from Thursday said al-Qaida may be considering attacks that use improvised explosives packed in vehicles, similar to the "attempted attack on Times square" by Faisal Shahzad in May 2010.

Al-Qaida may be aiming to avenge the death of Osama bin Laden and other key terror figures, the bulletin said.

Officials were also investigating information about two construction vans stolen along roads in Queens on Sept. 1 or 2. A government official says the theft may have been from somebody working with the van, theorizing that it may be an inside job.

The alleged terror threat by al-Qaida against New York City and Washington D.C has brought additional police officers, vehicle checkpoints, and subway bag checks.

That level of security is expected to stay in place through the Monday evening rush hour, the NYPD said.

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